My mom & dad have their birthdays on the same day (Dec. 2) and coincidentally so does my mom's sister & her husband (Mar. 29). For years I believed that when you grew up & got married that you automatically had the same birthday as your mate. I think I was probably about 9 or 10 when they finally clued me in.
A childhood belief about birthdays

others believed things like this
- When I was little I used to think that as soon as you were married the woman automatically got pregnant.Anon
- I used to think you only grew on your birthday.Anon
- I used to believe that people only grew, physically and in age, on their birthday. — Aretia
- I used to think you couldn't die if you didn't have shoes on. Coincidentally, so did one of my friends.— Casey
- i used to think that if i didn't celebrate my birthday each year my younger sister would catch up with me— q
- My sister uesd to think that you did a whole year's worth of growing in the night before your birthday.— Richard Davies